Surface coating of ferrimagnetic and/or ferromagnetic nanoparticles with desired functionality and controlled magnetic properties is critical to the development of magnetic nanomaterials for high density recording media as well as biomedical applications. A significant challenge to utilizing magnetic nanoparticles for materials applications is the inherent aggregation of nanoparticles that takes place as a result of magnetic interparticle attractions. Strong magnetic nanoparticle interactions result in poor nanoparticle dispersion in solvents. Well-dispersed samples of magnetic nanoparticles are desirable for processing the particles from solution to form, for example, magnetic tape media. Magnetostatic exchange coupling interactions are highly dependent upon interparticle distances, thus, the interactions can be minimized by introducing a non-magnetic shell around the nanoparticles.
Among the various ferrite materials used in magnetic recording media applications, ferrimagnetic cobalt ferrite (CoFe2O4) nanoparticles (>˜16 nm) with inverse spinel structures are of particular interest. These nanoparticles, which can be synthesized via colloidal methods, possess excellent chemical stability and mechanical strength as well as magnetocrystalline anisotropy and moderate saturation magnetization.
The solution phase synthesis of CoFe2O4 nanoparticles with uniform size and morphology has progressed significantly during the last decade. One of the most commonly used solution phase methods for synthesizing CoFe2O4 is the thermal decomposition of Fe(acac)3 and Co(acac)2 precursors in the presence of oleic acid surfactants in a high boiling point solvent, such as benzyl ether. With this method, oleic acid surfactants protect the resulting CoFe2O4 nanoparticles and afford the nanoparticles solubility in nonpolar solvents, such as hexane. The magnetic properties of CoFe2O4 nanoparticles synthesized in this way may be changed from superparamagnetic to ferrimagnetic at room temperature by altering the size and shapes of the nanoparticles. The successful synthesis of CoFe2O4 nanoparticles using the oleic acid surfactant method is therefore two-fold, depending on: (i) the ability to modify the surface of the nanoparticles by controlling shell thickness, colloidal stability, and surface functionality; and (ii) the ability to control the composition, shape, size, and magnetic properties of the nanoparticles.
The successful synthesis of magnetic nanoparticles by the oleic acid surfactant method, however, does not ensure the successful industrial application of the nanoparticles. A disadvantage of oleic acid surfactant magnetic nanoparticle synthesis is the instability of the resulting magnetic nanoparticles; specifically, as a result of strong magnetic forces, magnetic nanoparticles in solution have the tendency to irreversibly aggregate and ultimately precipitate from the solution. This aggregation of the magnetic nanoparticles renders the nanoparticles unsuitable for silica encapsulation.
The formation of silica core-shell nanoparticles is known to those experienced in the art. The most widely used silica coating method is the tetraethylorthosilicate (TEOS) method. With this method, the silica precursor TEOS is added to a mixture of nanoparticles in an ethanol/ammonia solution in order to grow the silica shell on the nanoparticle surface. While this method is suitable for nanoparticles, such as metal nanoparticles, quantum dots, and superparamagnetic particles, this method is not suitable for creating uniform silica shells around magnetic nanoparticles. Metal nanoparticles, quantum dots, and superparamagnetic particles are suitable for the TEOS method because they do not have the same interparticle magnetic forces that are present with magnetic nanoparticles. In this vein, magnetic nanoparticles are unsuitable for the TEOS method because the strong interparticle magnetic attractions of the magnetic nanoparticles cause irreversible aggregation of the nanoparticles, thus preventing the formation of a uniform silica shell around the individual nanoparticles.
As noted above, the inherent aggregation of magnetic nanoparticles and the formation of non-uniform silica shells around individual and/or clusters of the nanoparticles hinder the production of monodisperse magnetic nanoparticle samples for magnetic applications. Successful silicon encapsulation of magnetic nanoparticles thus requires a way to inhibit aggregate formation prior to growth of the silica shell.